How can I make dog treat icing that hardens for decorating treats?

To make dog treat icing that hardens for decorating treats, you can mix powdered sugar with a small amount of water or milk until you reach a thick consistency. You can also add a small amount of corn syrup or honey to help the icing harden. Once you've decorated the treats, let them sit at room temperature for a few hours to allow the icing to harden before serving them to your furry friend.